Widely utilized in the automotive field are diagnostics decoders for troubleshooting electrical or electronic units. These are aimed at obtaining as much detailed information as possible about malfunctions occurring within a unit, and this demand does bear on the individual electronic devices provided in the unit.
Accordingly, it would be useful, especially where the ratio of new functionality-to-silicon area of an integrated circuit chip, is an advantageous one, if the individual devices can be provided with full self-diagnosis capabilities. This would simplify the program software of the vehicle microprocessor, relieving it, in the instance of a bridge type of driver circuit, of the need to have the diagnostics "word" read at each of the two drive steps, if an information about short-circuit trouble is to be issued in detail.
The internal diagnostic information of the device can be stored; the storage operation may be carried out concurrently upon sensing a possible malfunction if an alarm terminal to the microprocessor is provided (in which case, a malfunction read out to the microcontroller would be the one sensed first), or alternatively, be triggered by a request for the diagnostics "word" from the microprocessor, where the alarm pin is not there (in which case, a malfunction read out to the microprocessor would be the one sensed last).
Almost all of the current bridge-type drive devices provide diagnostics for short-circuits to the battery, short-circuits to ground, and an open load, but not for short-circuits across the load.
There exist two methods which are more frequently employed to provide such diagnostics, as follows:
1. Short-circuits to the battery and to ground are located by means of two comparators, which compare the voltage drop across resistive elements through which the load current is flowed with a reference threshold. The short-circuit diagnostic procedure is triggered by this threshold being overtaken. The open-load condition is provided by a comparator comparing the drop across a resistive element through which the load current is flowing with a reference threshold, the open-load diagnostics being triggered after the open-load condition has been sensed through a predetermined time period in some cases synchronized to the drive phases (the diagnostics would be invalid during the time periods when the bridge is disabled because such time periods are usually utilized to cancel it out.
2. Shorts to the battery or to ground are again located by the above method, but the short-circuit diagnostics is only triggered on after the reference threshold has been overtaken through a definite time period. This filtering interval, during which a current limiter circuit may be operated, is necessary to avoid any disturbance having to be taken account of, especially during the switchover phases when cross conduction may occur due to either a malfunction in the non-overlap intervals or to a delay in turning off the power transistors of the bridge. The filtering interval allows the shorted-load condition to be ascertained, which condition would be encoded, however, as a generic abnormal event. The open-load condition would only be sensed with the bridge disabled; accordingly, it can only be ascertained by forcing the microprocessor to periodically disable the driver circuit. In addition, no filtering of any kind is applied to that information, so that any disturbance to the open-load sensing would propagate asynchronously through the diagnostics. A device which utilizes this method is a TLE5203 chip marketed by Siemens.